Abstract: A refractory lined firebox is encased in a low profile furnace shell of generally rectangular configuration. Mounted on top of the firebox within the furnace shell is a heat exchanger which provides alternating passageways at right angles to one another through which flue gas and air to be heated flow in heat exchange relationship. The heat exchanger and firebox are so related that the axes of the passageways are inclined from the horizontal and the flow through them is generally upward as well as lateral. The furnace includes special combustion air ducts for use in combustion of the solid fuel. The furnace is also equipped with a back-up liquid fuel burner and a control system which activates and operates the liquid fuel burner in response to total consumption or burn out of the solid fuel in the firebox.